1. Field of the Invention
The present invention relates to an integrated optical displacement sensor by use of laser beams, and more specifically, an apparatus for measuring a fine displacement of a sample.
2. Description of the Related Art
Generally, optical sensors which measure a displacement of a sample based on an optical interference method are most widely used because the wavelength of a light beam can be used as a reference in these sensors. In the optical interference method, a measurement laser beam irradiated on a sample, and a reference laser beam irradiated on a surface which is used as a reference, are interfered with each other to measure a displacement in accordance with the configuration (projection and recess) of the sample. Some examples of the method which do not operate based on the optical interference technique are an optical lever technique, utilizing the widening angle of an optical fiber.
For example, in the optical cantilever method, a laser beam output from a laser is made incident obliquely on a sample, and its reflection light beam is received by an optical detector such as a beam split sensor. Therefore, the position of the light beam incident spot detected by the optical detector moves in accordance with the displacement of a sample in the surface normal direction, or the variance of the tilting angle with respect to the incident light beam. Accordingly, the output of the optical detector varies, and based on this variance, the displacement of the sample can be measured.
In the case where a beam-split photodetector is used as the optical detector, the displacement of a sample can be measured by monitoring the value (error signal) obtained from the calculation formula (A-B)/(A+B) where A and B are voltages output in accordance with the quantity of light made incident on of the respective sensors. The name, optical lever, is originated from cantilevers, which are samples to be measured. The displacement magnification m, which is an index of the sensitivity of the measurement of displacement, is proportional to L/l as below: EQU m.infin.L/l (1)
where L is a distance and between the cantilever and the optical detector, and l is a length of the cantilever.
In this conventional optical displacement measurement method, the longer the distance D between a sample the optical detector, and the larger is the displacement magnification m, as mentioned. Therefore, the longer the value L, the finer is the measurement of displacement. As a result, the size of the displacement measure inevitably becomes large.
The optical displacement sensor is applied to a measurement device such as an atomic force microscope (AFM). In this device, displacement of a cantilever which displaces in accordance with displacement of a sample is measured by the optical displacement sensor, and thus the displacement of the sample is indirectly measured.
The AFM has been proposed as a microscope for observing an insulating sample at an accuracy of atomic order by use of elemental techniques, including the servo technique, of the scanning tunnel microscope (STM) invented by Binning and Rohree et al. (U.S. Pat. No. 4,724,318: IBM, G. Binning, Device and Method of Forming the Image of the Surface of Sample). With the AFM, it is easy to observe an insulating sample, which is difficult to measure by conventional STMs.
The structure of an AFM is similar to that of an STM, and the AMF is regarded as a type of the scanning probe microscopes. According to the AFM, a cantilever having a sharply projecting free end (probe) is arranged such that the probe faces close to a sample, and movement of the cantilever caused by the interacting force between atoms located at the end of the probe and atoms of the sample, is electrically or optically measured. Further, while measuring the movement, the sample is scanned in the X, and Y directions to change the relative position of the sample with respect to the probe of the cantilever, and thus the three-dimensional configuration data of the sample can be obtained.
Such a high-resolution measuring device requires a highly sensitive displacement sensor to achieve a high resolution, and at the same time, the size of the device must be reduced to control mechanical vibration of the measuring device itself. However, the conventional techniques are not fully satisfactory as regards the above-mentioned points.